(1) Field of the Invention
The invention relates to the general field of ambient lit Liquid Crystal Displays, more particularly to the design of the reflective layer.
(2) Description of the Prior Art
First generation liquid crystal displays involve a number of layers, the outermost being a pair of crossed polarizers. In the most commonly used configuration, the polarizers are arranged so as to have their optic axes orthogonal to one another. That is, in the absence of anything else between them, light passing through the entrance polarizer would be blocked by the exit polarizer, and vice versa.
Conducting lines running orthogonal to, and insulated from, one another are located on the inside surfaces. These lines are connected at their intersections through Thin Film Transistors (TFTS). The TFTs allow voltage, separately applied to a set of orthogonal lines, to be added together only at the intersecting position which will overlie a given pixel (or sub-pixel) of the display.
Sandwiched between, and confined there by means of suitable enclosing walls, is a layer of liquid crystal. Liquid crystals comprise long molecules, referred to as nematic, or thread-like. The orientation of these molecules, relative to a given surface can be controlled by coating such a surface with a suitable orientation layer which is rubbed in the desired direction just prior to bringing it into contact with the liquid crystals.
Thus, the molecules closest to the upper substrate would be oriented so as to lie in one plane while the molecules closest to lower substrate would be oriented to lie perpendicular to this plane. Molecules in between the two sets of pre-oriented molecules then arrange themselves so as to gradually change their orientations between these two extremes. Hence the term `twisted nematic` (TN) for such a configuration. A TN is optically active and will rotate the plane of any polarized light that traverses it so that polarized light that was formed and oriented as a result of passing through an entrance polarizer will be rotated though an angle of 90.degree. after traversing the liquid crystal and so will be correctly oriented to pass through the exit polarizer. Such a device is therefore normally on (transmits light).
An important property of TN is that, in the presence of an electric field (typically about 10 kV/cm.), normal to the molecules, said molecules will all orient themselves so as to point in the same direction and the liquid crystal layer will cease to be optically active.
At the present time colored LCDs are built in the same way as monochrome LCDs but their light has been passed through multicolor filters. The latter consists of a matrix of sub-pixel size regions on a common substrate, each region being a tiny color filter. The spatial locations of the different colored regions are known to the liquid crystal display control system which determines the amount of light that is allowed to pass beyond any given dot, thereby creating a color image.
To view the display, light is applied from outside the entrance polarizer and then viewed by looking through the exit polarizer. This implies an independent light source as part of the overall display, adding significantly to its power requirements. In general, it would be preferable to be able to view the display using ambient light alone. In principle this could be achieved by locating a reflecting surface on the inside surface of the exit polarizer and then viewing the display directly.
In practice, this arrangement does not work well with LCDs of the type just described because the polarizers absorb a significant fraction of the incident light, often in excess of 50%. As a result, a new generation of LCDs are currently being developed that do not require the presence of a pair of polarizers.
FIG. 1 is a schematic cross-section of a LCD that operates without the need for polarizers. The figure is taken from a paper by Mitsui et al. "Bright reflective multicolor LCDs addressed by a-Si TFTs" published in SID 92 Digest pp. 437-440. Liquid crystal 10 is confined between upper and lower substrates 11 and 12 respectively, including sub-pixel sized color filters 13 and 14 and transparent conducting electrode 15. An electric field may be selectively applied between 15 and any one of lower electrodes 16.
Selection of a particular electrode is effected by means TFTs located at the intersections of the orthogonal addressing lines, as already discussed above. Source/drain electrodes 17 contact amorphous silicon layer 19, current through which is controlled from gate electrode 18. Lower electrodes 16 contact the TFTs through via holes such as 5.
In the example illustrated in FIG. 1, the sub-pixel corresponding to filter 13 is off, while the the sub-pixel corresponding to filter 14 is on. Two TN spirals are shown below 13. The liquid crystal molecules are schematically shown as open ellipses such as 1. The black ellipses represent black dye molecules that have been added to the liquid crystal. Rather than being randomly oriented within their LC host, these dye molecules follow the orientations of the LC molecules, in this case a spiral. In this configuration, the dye molecules block the passage of light through them.
Continuing to refer to FIG. 1, the sub-pixel corresponding to filter 14 is on. That is, an electric field has been applied across the LC/dye mixture. This causes the LC molecules, such as 4, to line up with the field. Dye molecules, such as 3, follow the orientation of the LC molecules and, now, do not block the passage of light through them. This type of LCD is said to be in phase change guest host (PCGH) mode.
Since the display of FIG. 1 is free of light absorbing polarizers, it can be viewed in ambient light applied from above upper substrate 11. Reflection of the light occurs at the surface of layer 16. It is important to note that layer 16 must not be a planar surface. If it were, the display would behave as an optical interference filter and transmit only a limited range of optical frequencies. Thus it becomes necessary to ensure that reflection at the top surface of layer 16 be non-specular.
A number of approaches to providing non-specular reflecting surfaces for use in LCDs have been described in the prior art. For example, Mitsui et al. (U.S. Pat. No. 5,204,765 April 1993), as illustrated in FIG. 2a, deposit a layer of resin on substrate 21 and then pattern and etch it to form an array of tiny pedestals 22 sticking out from the surface. The pedestals are then subjected to a heat treatment which causes them to assume a rounded shape somewhat akin to a set of convex lenses. These are shown as 23 in FIG. 2b. A reflective metallic layer is then deposited over convexities 23.
A different method for forming the non-specular reflector has been disclosed by Mitsui et al. in U.S. Pat. 5,220,444 Jun 1993. A layer of oxide is first deposited and then etched in a manner that results in a roughened surface. The latter is then given a metallic coating.
Both the above methods involve extra steps. Additionally, the first method results in a surface that is non-planar but its non-specularity follows an even pattern (which is not desirable from an optical standpoint), while the non-specularity of the second method is hard to predict.